Helical waveguide formed from dielectric ribbon having symmetrically disposed conductive strips on opposite sides

ABSTRACT

A helix waveguide structure is adapted to printed circuit techniques. In particular, the wire helix of the prior art is replaced by pairs of helices printed on opposite sides of a low loss dielectric member. The structure is advantageously fabricated by printing conductive strips on both sides of a dielectric ribbon and winding the ribbon into helix form.

United States Patet [72] Inventor Stewart E. Miller Middletown Township, Monmouth County, NJ.

[21] Appl. No. 806,662

[22] Filed Mar. 12,1969

[45] Patented Apr. 6,1971

[73] Assignee Bell Telephone Laboratories, Incorporated Murray Hill, NJ.

[54] HELICAL WAVEGUIDE FORMED FROM DIELECTRIC RIBBON HAVING SYMMETRICALLY DISPOSED CONDUCTIVE STRIPS ON OPPOSITE SIDES 1 Claim, 2 Drawing Figs.

[52] US. Cl 333/95,

[51] Int. Cl 1101p 3/12,

1101p 3/08,1-I01p 3/18 [50] Field ofSearch 333/95,95 (A), 95 (S), 84 (M), 31, 31 (A), 98 (M), 98, 84, 3.000

[56] References Cited UNITED STATES PATENTS 2,704,829 3/1955 Clay 333/31 2,774,046 12/1956 Arditi et al 333/84(M) 2,848,696 8/1958 Miller 333/95 2,905,858 9/1959 Cutler 333/3l(A) 2,915,718 12/1959 Grieg et al. 333/96X 2,950,454 8/1960 Unger 333/95 3,047,822 7/1962 Lakatos et al. 333/95X 3,106,768 10/1963 Gent et al. 333/95X 3,407,366 10/1968 Dworkin 333/84(M) 3,411,116 11/1968 Boutelant 333/98 FOREIGN PATENTS 950,133 9/1956 Germany 333/95 Primary Examiner-Eli Lieberman Assistant Examiner-Wm. I-I. Punter Attorneys-R. J. Guenther and Arthur J. Torsiglieri ABSTRACT: A helix waveguide structure is adapted to printed circuit techniques. In particular, the wire helix of the prior art is replaced by pairs of helices printed on opposite sides of a low loss dielectric member. The structure is advantageously fabricated by printing conductive strips on both sides of a dielectric ribbon and winding the ribbon into helix form.

INVENTOR SEM/LLE/P ATTORNEY HELICAL WAVEGUIDE FORMED FROM DIELECTRIC RIBBON HAVING SYMMETRICALLY DISPOSED CONDUCTIVE STRIPS ON OPPOSITE SIDES This invention relates to electromagnetic wave transmission systems and, more particularly, to the transmission of the circular electric or TE mode of wave propagation over long distances in a guidedwave transmission system which either through choice or inherency does not follow a perfectly straight path.

The propagation of microwave energy in the form of TE waves in circular waveguides is ideally suited for the long distance transmission of wide band signals since the attenuation characteristic of this transmission mode, unlike that of all other modes, decreases with increasing frequency. However, one difficulty with this method of transmission is that the TE mode is not the dominant mode supported in a circular waveguide, and consequently energy may be lost to other modes also capable .of transmission therein. In an ideal waveguide which is perfectly straight, unifonn and conducting, the propagation of TE waves therethrough is undisturbed, but slight imperfections in the guide and especially curvature of the waveguide axis may excite waves of other modes and produce serious losses. These losses are attributed mainly to the fact that the bending of the guide produces a coupling between the desired TE and other transmission modes, mainly the TM TE and TE modes.

Recognizing that the coupling between these modes may be likened to the coupling between traveling waves on coupled transmission lines in that an exchange of energy will take place between the waves when they travel together at the same phase velocity of propagation, the prior art has provided a large number of devices for negotiating bends or turns in the guides. Thus, the phase velocity of the TM mode (which is normally equal to that of the TE mode) is changed relative to that of the TE mode, to increase the relative differences in their propagation constants and to reduce the effective coupling therebetween.

Of the several prior art devices operating according to this principle, one of the most practical is the helical waveguide such as is described in US. Pat. No. 2,848,696, issued to S. E. Miller on Aug. 19, I958. In essence, the waveguide comprises an elongated member of insulated conducting material, such as enameled wire, would in a circular helix. The helix is typically covered with an electrically lossy material and surrounded by an outer shielding jacket, such as a steel tube, to protect it from external mechanical and electrical influences. When the inside diameter of the helix is greater than 1.22 times the free space wavelength of the lowest frequency wave to be propagated and the spacing between the centers of ad-- jacent turns of the helix is less than 0.25 times the wavelength of the highest frequency to be propagated, the helix will propagate frequencies within this bandwidth in the circular electric TE mode. As is well known in the art, the undesired TM modes produce predominantly longitudinal wall currents and are seriously affected by the insulated separation between adjacent turns of the helix. On the other hand, the circular electric mode produces circumferential currents which are not appreciably interfered with by the helical structure. In addition, the longitudinal currents of the undesired TM modes are forced to flow through the lossy material resulting in increased attenuation for these modes. The field of the circular electric mode, on the other hand, does not penetrate through the space between adjacent conductors into the lossy material (provided the diameter of the conducting wire is comparable to or larger than the spacing between adjacent turns) because the two adjacent conductors act as a waveguide beyond cutoff. As a result of the difierent reactance and attenuation, the phase velocity of the TM mode in such a waveguide is significantly different from that of the TE, mode, and the power transferred between the two modes in a bend is substantially reduced.

One difficulty with helical waveguides, however, is the 0 sion. Moreover, as higher frequencies are utilized, the wires become even thinner. One possible approach to simplifying the fabrication of the helix is to print a plurality of thin conducting strips on a stronger dielectric tape and then wind the tape to produce the desired helix. Unfortunately, in order to prevent significant loss to the TE mode due to penetration into the lossy layer, the conductive strips must be thicker than can be conveniently made by conventional printed circuit techniques. As has been previously explained, the conductive layers should have a thickness comparable to or greater than the spacing between adjacent strips in order to prevent loss to the TE. mode. However, the minimum allowable spacing is limited by the requirement that the capacitance between adjacent strips be kept low. Consequently, in waveguides for the millimetric range, the conductor thickness should be on the order of a few mils while conventional printed circuit techniques are typically limited to conductors on the order of one mil or less.

SUMMARY OF THE INVENTION In accordance with the present invention, a new waveguide structure has been devised to permit the use of printed circuit techniques in the fabrication of helical waveguides. In particu lar, the wire helix of the prior art is replaced by a double helix which can be advantageously provided by printing pluralities of pairs of thin conductive strips on opposite sides of a low loss dielectric ribbon.

BRIEF DESCRIPTION OF THE DRAWINGS The nature of the present invention and its various features will appear more fully upon consideration of the illustrative embodiments to be described in detail in connection with the accompanying drawings in which:

FIG. 1 is a perspective view of a printed circuit tape useful in fabricating a helical waveguide in accordance with the invention; and

FIG. 2 is a partially cross-sectional view of a typical helical waveguide in accordance with the invention.

DETAILED DESCRIPTION FIG. 1 is a perspective view of a printed circuit tape useful in fabricating a helical waveguide in accordance with the invention. It shows a plurality of thin strips 10 of conductive material such as copper disposed on opposite parallel sides of a flexible, low loss dielectric ribbon such as mylar tape. The conductive strips are substantially coextensive across the dielectric. As in the prior art the center-to-center spacing, D, between adjacent conductive strips 10 is less than one-quarter wavelength. For millimetric waves, this spacing is typically on the order of 4 mils. The spacing, S, between the edges of adjacent strips is determined by the desired capacitance and is typically on the order of a mil. The thickness, T, of the strips can be chosen for convenience of fabrication. Advantageously the strips are fabricated by printed circuited techniques and the thickness is on the order of 0.1 mil. The thickness T of the dielectric ribbon 11 is comparable to or larger than the spacing S but less than one-half wavelength of the highest frequency wave to be propagated. For millimetric waveguide this can be on the order of 3 mils. The length L of the dielectric tape is determined by the maximum allowable pitch angle in the helical waveguide. Typically it can be at least enough to hold two or three conducting strips.

As was previously stated, this printed circuit tape can be fabrication of the helix. Typically, the helix is formed by windconveniently fabricated by using printed circuit techniques to produce the conductive strips. The tape can then be wound into a helical waveguide using substantially the same processes as used in the prior art with the tape substituted for a single strand of wire. Since the tape is stronger than a single thin strand of wire, breakage is less likely; and since the tape carries more than one conducting strip, the number of turns required to produce a given length of helix is proportionately reduced. In addition, since much thinner conductors are used, there is a saving of this material.

FIG. 2 is a partially cross-sectional view of a typical helical waveguide structure made in accordance with the invention. The waveguide comprises a helix 20 of printed circuit tape as described in FIG. 1, an electrically lossy layer 21, i.e. typically a material having a resistivity on the order of one ohm-centimeter, surrounding and bonded to helix 20 and an outer shielding jacket 22 to protect the helix from external mechanical and electricalinfluences. The conductive strips form a helical waveguide. In addition little or no TE mode energy propagates through the spaces between adjacent conductive strips. The adjacent inner diameter strips and outer diameter strips behave as a waveguide surrounding the space between them. Thus, so long as the thickness T is comparable to or greater than the spacing S, the TE mode energy will see the space between adjacent strips as a waveguide beyond cutoff.

In all cases it is understood that the above described arrangements are illustrative of a small number of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

lclaim:

l. A transmission medium for guiding wave energy in the circular electric mode of wave propagation within a given band of frequencies comprising:

an elongated dielectric ribbon member having a plurality of conductive strips symmetrically located on opposite surfaces thereof; and

said ribbon and said strips disposed in a substantially helical configuration to form a helical waveguide;

characterized in that:

said strips are conductively insulated from each other and have a center-to-center spacing of less than onequarter wavelength of the highest frequency of said wave energy;

the thickness of said ribbon is comparable to or greater than the spacing between the edges of adjacent strips, but less than one-half wavelength of said highest frequency wave energy; and

in that an electrically lossy jacket overlays said helically wound ribbon and strips. 

1. A transmission medium for guiding wave energy in the circular electric mode of wave propagation within a given band of frequencies comprising: an elongated dielectric ribbon member having a plurality of conductive strips symmetrically located on opposite surfaces thereof; and said ribbon and said strips disposed in a substantially helical configuration to form a helical waveguide; characterized in that: said strips Are conductively insulated from each other and have a center-to-center spacing of less than one-quarter wavelength of the highest frequency of said wave energy; the thickness of said ribbon is comparable to or greater than the spacing between the edges of adjacent strips, but less than one-half wavelength of said highest frequency wave energy; and in that an electrically lossy jacket overlays said helically wound ribbon and strips. 